In the field of horticulture one of the most common practices involves starting all sorts of plants, including trees, in cells provided either singularly or combined in multi-cell seed trays. Starter cells give seedlings a chance to sprout and grow in controlled conditions to a point where the new plant can be successfully transplanted to a more permanent growing environment such as out-of-doors gardens, landscapes, commercial fields, and so on.
Before transplanting a new plant from a starter cell to a more permanent environment it is desirable to have as many roots already initiated and established under the plant as possible. Roots grow from their tips, and typically follow the path of least resistance. The greater number of roots a starter plant has, the better chance it has of successfully surviving and thriving under transplant conditions.
A problem with starter cells found in the conventional art is that they are not designed to optimally prevent twisting of roots that happens within typical starter cells as a result of the root tips growing along paths of least resistance.
A twisted root ball or plug is a mass of a comparatively small number of usually abnormally large roots that are twisted generally about one another, typically forming a spring-like twisted root configuration. Twisted root balls hinder establishment of a host starter plant after transplant, in some cases, thwart the development of the plant, causing later, sometimes much later, failure. The twisting action of the roots in the starter cell causes inhibition in new root growth, because newer roots must establish above the ball and must grow around the obstacle. As a result a twisted root ball has fewer total roots established. Twisted roots also continue to grow in a twisted fashion after transplant, causing further damage to the plant. The twisted roots are usually weak and shallow water roots that are not beneficial to new plant establishment. The shallow establishment and fewer roots associated with a twisted root ball also cause the plant to be susceptible to wind after transplant, leading to plant blow-over.
Another critical component of starting plants in a starter cell is termed air pruning in the art. Simply put, air pruning requires an opening to be provided at the bottom of the cell so that root tips growing beyond the opening die from exposure to air, thereby promoting further root growth behind the point of trimming. For proper air pruning to occur within a starter cell, important relationships between the diameter of the cell, the slope of the cell and the design of the opening of the cell have to be provided during design and manufacture of the cell.
Typically, many prior art cells and trays are rigid or semi-rigid products molded from plastic in other than vacuum forming operations. It is, however, desirable to vacuum form seed trays by heating sheets of thin-walled material and pulling portions of the sheets into molds by vacuum, instead of using injection or other molding techniques, because vacuum forming is typically less expensive. While rigid and semi-rigid molded cells can rather easily have bottom openings designed for adequate air pruning, vacuum formed cells are more pliable, being created from thin plastic sheet, and cannot economically be molded with complete openings in the bottom for air pruning purposes.
What is clearly needed is a vacuum-formed seed tray comprising cells having a functional features to solve the limitations of prior-art cells mentioned above. Such a tray would be less expensive to manufacture and would promote better root growth and success percentages for starter plants under transplant conditions.